Caliper gauges with "floating heads" on one or both sides of a traveling sheet are described in the U.S. Pat. Nos. 1,946,924 Allen, Re. 26,206 Knobel, 3,321,838 Albertson and 4,107,606 Typpo. More commonly, present commercial sheet caliper gauges either contact the sheet on both sides or the sheet is supported on one side by a contacting roll or plate while a floating head is used on the other side, as described, for example, in the U.S. Pat. No. 3,855,524 Crawford.
However, many sheet materials such as certain papers are often highly abrasive and may quickly destroy sheet-contacting elements. Rolls are subject to various out-of round conditions and bearing wear. Sheet materials may be sticky, or contain gummy substances that will cause rapid buildup of deposits on contacting parts. In U.S. Pat. No. 3,528,002 Dunlavey it is proposed to place noncontacting air bearing support members under the sheet while using a floating head on top of the sheet for caliper measurement, but the mere provision of an air film support for the sheet cannot ensure a constant distance between the traveling sheet and the distance-measuring element that is an essential part of the caliper gauge.
It is apparent that the use of dual floating heads has the advantage of providing a substantially noncontacting measurement, and the use therewith of a proximeter to measure the distance separating the two floating heads by magnetic or magnetoelectric coupling through the sheet has the advantage of eliminating the effect of runout in the traversing mechanism that constrains and guides the heads for measurement at various points across the width of a wide sheet of material, as explained in detail in U.S. Pat. No. Re. 26,206 supra. However, up to the present time the desired accuracy and reliability of the dual floating head instruments have apparently not been realized, and this may be due to the fact that the problems of eliminating friction and extraneous mechanical biasing forces had not yet been solved. The sources of friction are apparent from the structures of many prior art devices. The mechanical biasing forces are less obviously produced by various springs, magnets, tubes and wires that have been used, for example, to provide support and power for operating the caliper gauge devices and/or for transmitting output signals therefrom.
The ability to provide noncontacting measurements is a very important attribute of many other types of gauges for measuring other sheet properties. A beta radiation gauge, for example, may employ a radioactive source positioned on one side of a traveling sheet to direct beta rays into the sheet, while a radiation detector on the other side detects the beta rays penetrating the sheet to initiate a response indicative of the weight per unit area property of the sheet. A light source on one side of the sheet and a photodetector on the other may be used to measure the formation property by detecting the nonuniformity of light transmission through the sheet in neighboring small areas of the paper surface. In another arrangement, a visible light source and a photodetector may be used to measure the opacity property of the paper.
In many such other sheet property measuring instruments, as well as in caliper gauges, the basic gauging elements can be mounted in floating heads, or sensor and target bodies, using gas films, for example, to maintain the gauging elements at constant distances from the opposite sheet surfaces, in a manner suggested by the proposals for the glossmeters, or color and brightness sensors disclosed in the U.S. Pat. Nos. 3,890,049 Collins and 4,277,177 Larsen.
In diverse electronic and electromechanical arts, it is known to place measuring transducers and associated signal generating apparatus aboard small moving or movable bodies, using "wireless" techniques to transmit signals from, and/or power to, the apparatus. U.S. Pat. No. 3,619,612 Belke, for example, discloses a strain gauge bridge circuit aboard a high speed turbine impeller. The bridge circuit is powered by solar cells energized by a stationary light source, and the amplified unbalance signal from the bridge controls the intensity of light from a light emitting diode. The light from the diode is transmitted from the rotating impeller to a stationary photodetector that provides the strain gauge data to a recording device. Other wireless techniques are disclosed in the U.S. Pat. Nos. 3,046,792 Morgan, 3,174,341 Sudo, 3,246,308 Matthews, 3,336,795 Arakawa, 3,656,132 Brumbelow, 3,876,998 Richter and 4,025,912 Rice. In other U.S. patents, U.S. Pat No. 3,479,511 Clerc discloses wireless transmission of the ionization chamber measurement produced by a traversing gauge for traveling sheet materials, and U.S. Pat. No. 3,828,248 Wennerberg discloses a traveling sheet caliper gauge wherein the spacing between two sheet contacting members determines the frequency of an oscillator, which frequency is then converted to a caliper measurement.